The ability to produce aluminum titanate honeycombs that are extruded to shape (i.e., not machined to a final dimension) is dependant upon suitably controlling the variability in how much the filter shrinks (or grows) during the sintering or firing process. Increasingly stringent diesel particulate filter (DPF) contour specifications require careful control of the shrinkage of the extruded green honeycomb.
Methods to control the extent of shrinkage variability in ceramic honeycombs include calcining and/or milling/comminuting of the batch raw materials to a defined particle size distribution prior to extrusion into the honeycomb structure. For example, in silicon carbide honeycombs, altering the silicon content has been shown to affect the shrinkage behavior. Taruta, et al., “Influence of Aluminum Titanate Formation on Sintering of Bimodal Size-Distributed Alumina Powder Mixtures,” J. Am. Ceram. Soc., 80 [3], 551-56 (1997) the entire disclosure of which is incorporated herein by reference, showed that shrinkage and pore size distribution can be modified and controlled by mixing coarse and fine alumina within the same composition. Wang, et al., “Microstructure of Ceramic Membrane Support from Corundum-rutile Powder Mixture,” Powder Technology, 168, 125-133 (2006), the entire disclosure of which is incorporated herein by reference, showed that pore size distribution (pore radius) can be modified through controlled changes in batch titania, which alters the final stoichiometry. U.S. patent application nos. 2010/0052200 and 2011/0053757, the entire disclosures of which are incorporated herein by reference, disclose that shrinkage variability can be controlled by controlling the particle size distribution of graphite and/or alumina, respectively. However, there is still a portion of shrinkage variability that has not been accounted for.
Accordingly, an effective way to minimize day-to-day shrinkage variability in the large scale production of honeycombs is desired such that relatively stringent filter contour specifications may be achieved.